Clear coats have become increasingly popular as coatings for painted articles such as automobile body panels. Not only do clear coats generally have excellent gloss, but they also generally offer enhanced resistance to etching and scratching. Color-plus-clear composite coatings are also widely utilized in the coatings art. They are particularly desirable where exceptional gloss, depth of color, distinctiveness of image, or special metallic effects are desired. Such coatings, however, require an extremely high degree of clarity in the clear coat to achieve the desired visual effect. As such, the clear coat of a color-plus-clear composite coating is especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the clear finish of the coating that cannot be rubbed out.
It is often difficult to predict the degree of resistance to environmental etch that a clear coat will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as high-solid enamels, do not provide the desired level of resistance to environmental etch when used as the clear coat of a color-plus-clear composite coating.
Many compositions have been proposed for use as the clear coat of a color-plus-clear composite coating, such as polyurethanes, acid-epoxy systems and the like. Many prior art systems, however, suffer from disadvantages such as coatability problems, compatibility problems with the pigmented base coat, and solubility problems, for example.
Coating finishes, particularly exterior coating finishes in the automotive industry, are generally applied in two or more distinct layers. Usually one or more layers of primer coating composition is applied to the uncoated substrate first, followed by one or more top coat layers, such as the clear coat layers referred to above. Each of the layers provides important properties toward the durability and appearance of the composite coating. The primer layer may serve a number of purposes, such as promoting adhesion between the substrate and the coating. Additionally, the primer coating may be applied in order to improve physical properties of the coating, such as corrosion resistance or impact strength, or to improve the appearance of the coating by providing a smooth layer upon which the top coat layers may be applied.
Attempts in the prior art to enhance strength, hardness, and etch resistance have utilized modified, crosslinkable acrylic, polyester and alkyd resins. For example, advances in liquid crystal technology have been adapted for the coatings industry, with considerable emphasis being placed on imparting amorphous polymers with liquid crystal character. In this regard, U.S. Pat. No. 5,218,045 focuses on incorporating favorable high T.sub.g characteristics to low T.sub.g resins through the use of mesogenic groups, those containing two or more aromatic rings linked in the para position, particularly para-hydroxybenzoic acid. Thus, covalently binding mesogenic groups to typical amorphous polymers, such as carboxylic acid functional acrylic resins and alkyd resins, has been purported to result in harder coatings.
U.S. Pat. No. 5,393,840 relates to generating a polymer of optimal qualities by blending phenol-functional polyesters and hydroxyl-functional acrylic copolymers. The invention focused on merging the qualities of both polymers, where polyesters have good impact resistance and acrylics have good weatherability. Furthermore, in using a phenol-functional polyester, acid-etch or environmental etch resistance was said to be imparted to polyester coatings.
Attempts have also been made to address the problem in the coatings art wherein the ether linkage formed during the cure of a standard hydroxyl functional polymer with melamine results in a relatively weak link which can cause degradation. In this regard, it has been proposed to produce a urethane linkage upon curing with melamine based on a carbamate/melamine cure system.
The present invention is directed to a significantly different approach to eliminate weak ether linkages by providing a resin, preferably an ethylenically unsaturated monomer or copolymer, and most preferably an acrylic resin, modified by para-hydroxybenzoic acid or substituted para-hydroxybenzoic acid which may be utilized in a curable crosslinkable composite coating composition. The prior art does not appear to disclose the use of a hydroxyl-functional ethylenically unsaturated monomer or copolymer resin that is modified by para-hydroxybenzoic acid and thereafter cured by an aminoplast crosslinker to form a carbon-carbon site ortho to the phenolic hydroxy group.
Thus, the present invention fulfills a need in the art for coating compositions and, more particularly, clear coat and primer systems that greatly reduce, and more preferably eliminate, degradation of film properties while simultaneously offering good weatherability and improved environmental etch resistance.